It has long been known that exhaust gases produced by combusting hydrocarbon and other fuels can contribute to atmospheric pollution. Exhaust gases typically contain pollutants such as nitric oxide (NO) and nitrogen dioxide (NO.sub.2), which are frequently grouped together as NO.sub.x, unburned hydrocarbons (UHC), carbon monoxide (CO), and particulates, primarily carbon soot. Nitrogen oxides are of particular concern because of their role in forming ground level smog and acid rain and in depleting stratospheric ozone. NO.sub.x may be formed by several mechanisms. First, the high temperature reaction of atmospheric oxygen with atmospheric nitrogen, particularly at adiabatic flame temperatures above about 2800.degree. F., forms "thermal NO.sub.x " through the Zeldovich mechanism. Second, the reaction of atmospheric nitrogen with hydrocarbon fuel fragments (CH.sub.1), particularly under fuel-rich conditions, forms "prompt NO.sub.x ". Finally, the reaction of nitrogen released from a nitrogen-containing fuel with atmospheric oxygen, particularly under fuel-lean conditions, forms "fuel-bound NO.sub.x ". In typical combustors, atmospheric oxygen and nitrogen are readily available in the combustion air which is mixed with the fuel.
Various combustor strategies can be employed to decrease the formation of thermal and prompt NO.sub.x. For example, a combustor may be configured to operate uniformly fuel-lean, that is, at an equivalence ratio less than 1.0. The equivalence ratio is the ratio of the actual fuel/air ratio to the fuel/air ratio required for stoichiometric combustion. An equivalence ratio greater than 1.0 indicates fuel-rich conditions, while an equivalence ratio less than 1.0 indicates fuel-lean conditions. Fuel-lean operation lowers adiabatic flame temperature, resulting in lower thermal NO.sub.x formation, and decreases the formation of prompt NO.sub.x.
In lean premixed/prevaporized (LPP) combustion fuel and air are mixed upstream of a combustor to form a well-mixed, uniformly lean, gaseous fuel/air mixture which is then combusted at a uniformly low temperature and low gas residence time. A well-mixed fuel/air mixture permits leaner combustion and, therefore, lower adiabatic flame temperatures, resulting in lower thermal NO.sub.x production. A uniformly lean fuel/air mixture has a uniformly low CH.sub.i concentration, that is, no regions of high CH.sub.i concentration, decreasing the production of prompt NO.sub.x. Both NO.sub.x reduction mechanisms rely on achieving thorough fuel/air mixing upstream of the combustor. Attempts to fully realize the low NO.sub.x potential of LPP combustion, however, have had only limited success due to autoignition occurring during fuel/air mixing and poor flame stability in the combustor at low equivalence ratios.
Autoignition of the fuel/air mixture during mixing liberates the fuel's chemical energy at a point at which it cannot be readily used and creates the potential for severely damaging the combustor. The tendency of any particular fuel/air mixture to autoignite is measured by the ignition delay time, which is the length of time at which the fuel/air mixture can be held at a particular temperature without autoigniting. Ignition delay times generally decrease with increasing mixture temperature. For example, a typical gas turbine engine fuel, Jet A, has an ignition delay time of about 0.1 millisecond (msec) at 1200.degree. F. and 10 atmospheres (atm), a typical fuel/air mixture temperature for advanced gas turbine engines. The fuel/air mixture temperature is dictated by the temperature of the air, which is usually hot compressor discharge air. A short ignition delay time makes it very difficult to thoroughly mix the fuel and air to form a well-mixed, uniformly lean fuel/air mixture.
Poor flame stability is a problem inherent in many lean combustion schemes. As the equivalence ratio is decreased, flame stability decreases rapidly. Very simply, at low equivalence ratios the flame is blown out. The lowest equivalence ratio at which combustion can be sustained is known as the lean combustion limit. In order for LPP combustion to achieve its full NO.sub.x reduction potential, the problems of autoignition and flame stability must be solved.
Accordingly, what is needed in the art is a method and system for LPP combustion which decreases the autoignition tendency of the fuel/air mixture and extends the lean combustion limit.